NEW YORK (FORTUNE) -
Forget fat and carbs - the key to weight loss could lie in micronutrients like amino acids, according to groundbreaking new research that will be of interest to food and pharmaceutical companies alike.

The study, published in the May 12 issue of Science, found that the amino acid leucine, a basic building block for proteins, also regulates the neural circuits that control appetite. Leucine does this by activating an intracellular pathway called mTOR, which acts as a sort of fuel sensor for our body's cells, determining their rate of growth. Basically, the leucine tricks the cell's fuel sensor into thinking it's full.

In the study, conducted over two years by the University of Cincinnati's Genome Research Institute, rats injected with leucine ate less and gained 60 percent less weight than a control group.

That's promising news for the two-thirds of American adults who are overweight or obese, according to federal data. It is also a potential gold mine for the R&D arms of big food and drug companies. But don't expect leucine-laced Cheerios anytime soon: The researchers didn't feed leucine to the rats - they injected it straight into their brains.

"We do not know whether ingesting more leucine is a way to lose weight," says professor Randy Seeley, the lead researcher and associate director of the university's Obesity Research Center.

Brazilian researchers have found that leucine supplements reduced body fat in rats, but Seeley believes that could simply be explained by the rats not liking the taste of the leucine in their diet. (Leucine is found naturally in meat and dairy protein.)

In any event, the work of Seeley and his Cincinnati colleagues has profound implications that could spur innovation on the part of companies large and small. Understanding how different micronutrients affect specific metabolic functions, Seeley argues, could allow researchers to come up with a "rational scientific design" for our diets.

Indeed, the Department of Defense has already asked Seeley's team to look into diets for soldiers that could be tailored to the metabolic demands of various phases of a mission.

Seeley's team has also done obesity research for Procter & Gamble, Merck and Eli Lily, and is currently working with some biotech startups.

Seeing how lucrative the diet business has becomeódiet plan peddler NutriSystem (Research), for example, was one of the top performers on the NASDAQ last yearóthere's sure to be a hungry audience for any products based on these findings.

"The next challenge will be to determine whether novel therapies for metabolic disease will emerge from pharmacological or nutritional exploitation of these insights," says Jeffrey Flier, a professor of medicine at Harvard Medical School, who wrote an editorial accompanying Seeley's research.

The study also found a novel way that leptin acts in the brain. The hormone leptin has been under the microscope since its discovery in 1994, thanks to its role in regulating appetite and metabolism. "We found that leptin acts, in part, by turning on mTOR in the brain," says Seeley. "No one knew this before." Overall, though, Seeley says leptin has not lived up to its initial promise as a fat fighter.

Seeley, a neuroscientist by training who weighs himself every day, has always been interested in the brain and behavior, "and eating is a very interesting behavior." How that behavior is influenced by our biological makeup is fertile ground for study.

"Asking folks to lose weight without changing our biology is hard," Seeley says, "because eating less is not something biology allows you to do."